This invention relates to an image reader which is capable of reading image data from an original that has a large number of gradations, for use in facsimile and digital copying machines and so forth.
A conventional image reader is provided with a charge coupled device (abbreviated to CCD hereinafter) image sensor which reads image data for a line using reading devices thereof, and sequentially outputs the image data which has been read by the CCD.
The operation of the conventional image reader will now be described.
The direction in which the image reading devices of the CCD image sensor are disposed is called a main scanning direction, while the direction perpendicular to this main scanning direction is called a sub-scanning direction. The CCD image sensor is supported so as to be capable of moving in the sub-scanning direction.
The image reader irradiates the surface of an original with light. The light reflected from the surface of the original is focused to form an image on the reading devices of the CCD image sensor by means of a rod-lens array. The reading devices of the CCD image sensor output voltages corresponding to the image data at each of a number of points of the original. The CCD image sensor sequentially outputs the data obtained by each reading device, in correspondence with scanning clock signals.
The output voltages are amplified to increase each data item obtained by the reading devices to a desired level by means of an amplifier. The amplified voltages are converted to obtain digital values in accordance with the levels of brightness of the reflected light, by means of an analog/digital converter.
Image data of a large number of gradations for a line can be obtained by the image reader by way of conducting the above operation over one line in the main scanning direction.
The carriage is then moved one line space in the subscanning direction so as to read image data of the next line. Image data of the entire surface of the original can be read by this image reader by way of repeating the above operation over the entire surface of the original.
The conventional image reader is not only used for reading drawings or text, but it is also used for reading photographs in magazines.
The conventional image reader, however, encounters a problem because it reads image data of an original such as a photograph by a CCD image sensor having a finite number of reading devices arranged in a row. The problem is raised by the density of reading lines of 4 lines/mm or thereabout in both the main scanning direction and the sub-scanning direction. Therefore, false signals, which are moire fringes, are generated by interference between the half-tone dots used to form the original photograph in the magazine and the reading lines of the CCD image sensor. This interference is caused by the arrangement of both the half-tone dots and the reading lines which are disposed in a distributed manner. As a result, the quality of the read half-tone dot prints read by the conventional image reader is excessively deteriorated
In order to overcome this problem, an image reader is known wherein the width of an aperture which is the effective width for reading is increased with respect to the densities of the main scanning reading lines and the sub-scanning reading lines.
Another solution to the problem is provided by another image reader in which the densities of the main scanning reading lines and the sub-scanning reading lines are arranged to be sufficiently large with respect to the spatial frequency of the original print to be read.
The latter image reader, however, raises a problem in that the necessary density of reading lines increases as the density of the half-tone dots increases. As a result, the number of reading devices of the CCD image sensor becomes very large, and the number of image data items also becomes very large. The operating speed of the scanning clock must be increased, and the speed of the processing system must also be increased, so that, as a result, the image reader system becomes expensive.
The former image reader which has a CCD image sensor with a wider aperture, suffers from the following problem. That is, the generation of false signals cannot be prevented by enlarging the width of the aperture when the density of the half-tone dots exceeds 2 lines/mm because the density of the reading lines is 4 lines/mm.
A conventional method which sacrifices the accuracy of reading to a certain degree so as to prevent moire fringes is known. The conventional image reader which uses a CCD image sensor with a wider aperture averages image data of four or nine adjacent points. The image reader in which image data of nine points is averaged to obtain arithmetic mean data will now be described.
The image reader reads image data of reading points over a plurality of lines, and temporarily stores the image data which has read. The image reader then reads data of a reading point and the eight points surrounding that point. Each data item is multiplied by coefficients to which weighting corresponding to the positional relationship of the reading points have been added. The data which has been weighted in this manner is then added, and is averaged to calculate arithmetic mean data of the reading point.
This type of image reader takes too long to output data after reading, and, as a result, a problem is raised in that the reading speed of the system cannot be raised because the image reader has a step in which data which has been read is temporarily stored in a memory, and this stored data is read and averaged.